In the typical exposure apparatus of this type, a light beam emitted from a light source is guided through a fly's eye lens (or microlens array) as an optical integrator to form a secondary light source as a substantive surface illuminant consisting of a lot of light sources. Light beams from the secondary light source are incident to a condenser lens.
The light beams converged by the condenser lens illuminate a mask with a predetermined pattern thereon in a superposed manner. Light having passed through the pattern of the mask travels through a projection optical system to form an image on a wafer. In this manner the mask pattern is projected (or transferred) onto the wafer as a photosensitive substrate to effect exposure thereof. The pattern formed on the mask is of high integration and a uniform illuminance distribution must be achieved on the wafer in order to accurately transfer this microscopic pattern onto the wafer.
Furthermore, attention is being focused on the technology of forming the secondary light source of circular shape on the rear focal plane of the fly's eye lens and changing the size thereof to vary the coherency σ of illumination (σ value=diameter of an aperture stop/pupil diameter of a projection optical system, or σ value=exit-side numerical aperture of an illumination optical system/entrance-side numerical aperture of the projection optical system). In addition, attention is also focused on the technology of forming the secondary light source of annular shape or quadrupole shape on the rear focal plane of the fly's eye lens to improve the depth of focus and resolving power of the projection optical system.